High pressure electric discharge lamp

ABSTRACT

The arc tube of an arc discharge lamp has two main electrodes and a starter electrode adjacent to one of them, the starter electrode being electrically connected to the other main electrode through a ceramic capacitor. The capacitance and voltage rating of the capacitor are such as to provide sufficient capacitive reactance for limiting the current to the starter electrode and to function as a direct current blocking capacitor during normal operation of the lamp.

United States Patent Peterson [451 Dec. 19,1972- [54] HIGH PRESSURE ELECTRIC DISCHARGE LAMP Y [72] Inventor: Carl L. Peterson, Gloucester, Mass.

[73] Assignee: GTE Sylvania Incorporated [22] Filed: Jan. 22, 1971- 21] Appl. No.: 108,838

[52] US. Cl ..313/197, 315/60 [51] Int. Cl ..H0lj 17/00 [58] Field of Search ..3l5/60, 234; 313/197, 198

[56] References Cited UNITED STATES PATENTS 1/1938 Kassher ..3l5/234 X 4/1937 Knowles 2/1937 Watrous, Jr. ..315/234 Freeman ..3l5/60 X Gottschalk ..313/198 X Primary Examiner-Nathan Kaufman Attorney-Norman J. OMalley, Edward J. Coleman and Joseph C. Ryan r [57] 1 ABSTRACT I The arc tube of an arc discharge lamp has two main 5 Claims, 1 Drawing Figure HIGH PRESSURE ELECTRIC DISCHARGE LAMP BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to the field of high pressure are discharge lamps and is especially applicable to such lamps having a metallic halide fill.

' 2. Description Of The Prior Art High pressure are discharge lamps generally comprise an elongated arc tube containing an ionizable fill and having press seals at each end of the tube. Disposed within the arc tube are two main electrodes, one at each end. The electrodes are generally supported in the press seals and are usually connected to a thin molybdenum ribbon disposed within the press seal, the purpose of the ribbon being to prevent seal failures because of thermal expansion of the lead-in wire.

In order to facilitate starting of the arc discharge, that is, ionizing of the gas fill, a starter electrode is generally disposed in the arc tube adjacent to one of the main electrodes. Such an electrode is to initiate a glow discharge. The glow discharge provides electrons and ions which enter the electric field between the main electrodes and ionize the gas to the extent that the arc discharge occurrs. Once the arc has ignited, the resistance of the ionized gas between the two main electrodes is considerably reduced.

I-Ieretofore, the starter electrode was electrically connected through a current limiting resistor, typically about 40,000 ohms, to the same side of the power supply as the opposite main electrode.

During operation of the lamp, electrolysis between the starter electrode and adjacent electrode can occur in the press seal, if there is a direct current (DC) electric potential therebetween. Electrolysis can always be present, since the arc tube material, generally high silica glass or quartz, contains minute quantities of alkali metals as impurities.

Electrolysis current usually produces harmful results only when the starter electrode is at a positive DC potential with respect to the adjacent electrode; it can deteriorate, tothe point of failure, the high silica or quartz glass in contact with the molybdenum ribbon to which the starter electrode is connected. Even when the lamp is energized by an alternating current (AC) voltage, there can be a DC component of potential due to the fact that the main electrode is at a considerably higher temperature than the starter electrode. The relatively cool starter electrode collects electrons more readily than the relatively hot adjacent electrode. This leads to a DC potential, with the starter electrode positive with respect to the adjacent electrode, that results in electrolysis.

In the prior art, one means used to eliminate a DC potential therebetween was a temperature sensitive switch, such as a U-shaped strip of bimetal, which, upon heating thereof, shorted the starter electrode lead-in wire to the adjacent electrode lead-in wire. A short period of time, say, about 30 seconds, was all that was normally required for the switch to heat up sufficiently to deflect and short the wires. As long as the wires were shorted and, thus, were at the same potential, no electrolysis could occur between :the two electrodes. However, electrolysis could occur during the period of time required for the switch to close.

During operation of the lamp, prolonged exposure of the switch to the heat emanating .from the arc tube could cause the bimetal to take a set in the stressed position, with the result that the switch could require progressively longer time intervals to close. In some SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an arc discharge lamp having improved means for preventing or substantially reducing the occurrence of electrolysis between the starter electrode and its adjacent main electrode.

A more particular object of the invention is to provide starting means for an arc discharge lampv which employs a minimum number of components for economically and reliably preventing or substantially reducing interelectrode electrolysis.

Briefly, these object are attained in an electricdischarge device having first and second main electrodes and a starter electrode disposed within an arc tube, with the starter electrode adjacent the first main electrode, by connecting a capacitor between the starter electrode and the second main electrode to provide the dual functions of current limiting and DC filtering.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is an elevational view of a high pressure arc discharge lamp in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawing an arc discharge lamp in accordance with this invention comprises an outer vitreous envelope or jacket 2 of generally tubular form having a central bulbous portion 3. Jacket 2 is provided at its end with a sealed re-entrant stem 4 through which extend relatively stiff lead-in wires 5 and 6 connected at their outer ends to the electrical contacts of the usual screw type base 7. Centrally disposed within jacket 2 is are tube 8 which is supported at its lower end by metal frame 9 which, in turn, is welded to lead-in wire 6. Attached to the upper end of arc tube 8 is metal frame 10, part of which frictionally engages the upper tubular portion of jacket 2 and stabilizes the position of arc tube 8.

Arc tube 8 is made of quartz, a high silica glass, although other types of glass having comparable or higher softening temperatures, such as alumina glass, may be used. Sealed in arc tube 8, at the opposite ends thereof, are main discharge electrodes 11 and 12 which l060ll 0602 are supported on lead-in wires 13 and 14 respectively. Each main electrode comprises a core portion which may be a prolongation of wires13 and 14 and may be prepared of a suitable electrode metal such as tungsten or molybdenum. The prolongations or wires 13 and 14 can be surrounded by tungsten or molybdenum wire heliires. a

An auxiliary starter electrode 15, generally prepared of tungsten, is provided at the lower end of arc tube 8 adjacent main electrode. 11 and comprises an inwardly projecting end of another lead-in wire.

The ends of the lead-in wires are welded to molybdenum ribbon connectors which are completely embedded within the press seal ends of arc tube 8. Relatively short molybdenum wires 16, 17 and 18 are welded to the ends of the molybdenum ribbon connectors and serve to convey current to electrodes", 12 and 15 respectively.

Wire 16' is electrically connected through metal frame 9 to lead-in wire 6 by means of nickel strip 19 connected between wire 16 and frame 9.

Wire 17 is electrically connected to lead-in wire by means of wires 20, 21 and 22 connected in series. Wire 20 is welded directly to wire 17 and wire 21 isa long thin wire extending from the upper portion of jacket 3 to the lower portion thereof.

In typical prior arc lamps, wire 18 was electrically connected to lead-inwire 5 through a 40,000 ohm resistor, which served to limit the current to starter electrode during normal starting of the lamp. In accordancewith the present invention, however, only a capacitor 23 is connected between wire- 18 and lead-in wire 5 (via a portion of wire 22), that is, capacitor 23 is electrically connected between starter electrode 15 and main electrode 12. Capacitor 23 is selected to have a capacitance value which provides, at the applied 60 hertz AC line voltage, a capacitive reactance sufficient to supplant the current limiting function formerly provided by the 40,000 ohms resistor. In addition, the DC voltage rating of capacitor 23 is selected such that it functions as a DC blocking capacitor to filter DC from the starter circuit during normal operation of the lamp. Further, in view of the relatively high voltages and ambient temperatures encountered in this application, capacitor 23 is preferably of the ceramic dielectric type with a glass casing. For example, in the operation of a 400 watt metal halide arc lamp, improved electrolysisfree performance was achieved by using as capacitor 23 a 0.02 microfarad, 1000 volts DC, ceramic capacitor, available from San Fernando Mfg, Co., 1501 First Street, San Fernando, Calif.

' Heat shield 25 is supported on frame 9 and is disposed below the lower end of arc tube 8 so as to shield capacitor 23 from direct heat radiation from arc tube 8. Glass sleeves 26 electrically insulate the wires passing through heat shield 25 to thereto. I

Arc tube 8 is provided with a filling of mercury which reaches pressures in'the order of one-half to several atmospheres during normal lamp'operation at temperatures of about 450 to 800C. The filling also includes an ionizable gas, argon, for example, at an approximate fill pressure of 25 torr. The filling also includes a halogen, except fluorine, and is preferably added in the form of an iodide of a suitable metal, such as sodium iii bmmn, a ballast ed AC voltage (typically 220 volts, 60 hertz) is applied to. lead-in wires 5 and 6. The reactance of capacitor 23 serves to limit the current in the circuit of electrode 15. In addition, however, capacitor 23 functionsto reduce the direct current in the starter circuit to a value low enough 'to prevent or substantially reduce the occurrence of electrolysis between electrodes 15 and 11 throughout all'phases of lamp operation. Normal ACoperation, of course, is unaffected by the insertion of capacitor 23, other than for desired current limiting. Thus, by employing a capacitor as described, the present invention significantly enhances lamp reliability by providing electrolysis-freeoperation, while at the same time minimizing the number of lamp components, and thus cost, by providing the additional function of current limiting.

What i claim is:

1. An alternating current electric discharge device comprising:

an arc tube containing an ionizable dischargesustaining filling; first and second main electrodes disposed within said arc tube;

a starter electrode disposed within said are tube adjacent to and dielectrically isolated from said first main electrode; a

and a capacitor connected between said starter electrode and said second main electrode. 1

prevent shorting 2. The electric discharge device of claim 1 wherein said capacitor has a capacitance value and direct current voltage rating selected to provide sufficient capacitive reactance for limiting the current to said starter electrode and to function as a direct current blocking capacitor during normal operation of said discharge device.

3. The electric discharge device of claim 1 further including a light-transmitting jacket enveloping said arc tube but spaced therefrom. 1

4. The electric discharge device of claim 3 wherein said capacitor is disposed between said are tube and said jacket.

5. The electric discharge device of claim 4 wherein said capacitor has a ceramic dielectric.

i t i l060ll 0603 

1. An alternating current electric discharge device comprising: an arc tube containing an ionizable discharge-sustaining filling; first and second main electrodes disposed within said arc tube; a starter electrode disposed within said arc tube adjacent to and dielectrically isolated from said first main electrode; and a capaCitor connected between said starter electrode and said second main electrode.
 2. The electric discharge device of claim 1 wherein said capacitor has a capacitance value and direct current voltage rating selected to provide sufficient capacitive reactance for limiting the current to said starter electrode and to function as a direct current blocking capacitor during normal operation of said discharge device.
 3. The electric discharge device of claim 1 further including a light-transmitting jacket enveloping said arc tube but spaced therefrom.
 4. The electric discharge device of claim 3 wherein said capacitor is disposed between said arc tube and said jacket.
 5. The electric discharge device of claim 4 wherein said capacitor has a ceramic dielectric. 